5-{[(3R)-1-Acryloylpyrrolidin-3-yl]oxy}-6-ethyl-3-({4-[4-(4-methylpiperazin-1-yl)piperidin-1-yl]phenyl}amino)-pyrazine-2-carboxamide (hereinafter sometimes referred to as “compound A”) is a compound represented by the following chemical structural formula. Compound A or a pharmaceutically acceptable salt thereof is known to be useful as an active ingredient of a pharmaceutical composition for treating cancer (Patent literature 1).

As compound A or a pharmaceutically acceptable salt thereof, Patent literature 1 discloses its free form in Example 54, and its monomethanesulfonate in Example 261, and discloses that the inhibitory action on an epidermal growth factor receptor (EGFR) mutant kinase has been confirmed.
Even today, in which the advancement of medical care is remarkable, especially for cancer, treatment satisfaction is low, and the contribution of further medicines is required. Providing stable medicines to medical professionals and people who need treatment of diseases and illness plays an important role contributing to the health of people around the world.
There are various destabilization mechanisms of drugs. There is a problem with the stability of the drug itself; in a pharmaceutical composition, particularly in a solid pharmaceutical composition, for example, there is a problem in the interaction between a drug and various pharmaceutical additives, or there are causes of instability of a drug in the manufacturing process; in a pharmaceutical composition, a drug reacts with moisture contained in pharmaceutical additives or the like (for example, Patent literatures 2 and 3); and the like. As described above, in terms of the nature of pharmaceuticals, it is extremely important to inhibit the generation of related substances, or the increase in the amount of related substances. However, a general method has not been established for the stabilization of drugs, and even at present, it has been sought for stabilizing a manner suitable for each drug.